Electrolytic capacitors



Feb. 1l? 1964 v J. BURNHAM 3,120,695

ELECTROLYTIC cAPAcI'roRs Filed Nov. 1'8. 1957 @1 /ass CEPR/wc: MPE E40 BZ/EQ;

United States Patent C) 3,120,695 ELECTROLYTIC CAPACITORS .lohn Burnham, 10960 Verano Road, Los Angeles, Calif. Filed Nov. 18, 1957, Ser. No. 697,159 4 Claims. (Cl. 29--25.31)

This application relates to the field of electrolytic capacitors. More specifically it relates to a method of making tantalum electrolytic capacitors, to a new electrolyte for use with electrolytic capacitors, and to tantalum electrolytic capacitors made by this method containing this electrolyte.

In order to understand the present invention and many advantages of it, it is considered necessary to set forth certain of the history of the field of electrolytic capacitors. Immediately prior to the past decade practically all coming an aluminum anode covered with an aluminum oxide dielectric lm, this anode being immersed in an electrolyte in which there was also immersed another electrode. Units of this type utilizing the valve metal aluminum are commercially made to be used at voltages up to about 500 volts. With them comparatively Weak electrolytes such as are formed by dissolving boric acid or various borates in polyhydric alcohols serving as solvents are commercially employed. Such electrolytes tend to decompose, as when an aluminum electrolytic capacitor is used at an elevated temperature, so as to produce gas.

.mercial electrolytic capacitors were manufactured utiliz- In addition, the conductivity of such electrolytes is not.

types of electrolytes which may be employed in these capacitors.

Because of the fact that tantalum is more resistant to chemical corrosion than aluminum, stronger electrolytes may be employed in tantalum electrolytic capacitors than may be employed with aluminum units. Mineral acids such as sulphuric acid and concentrated aqueous solutions of salts such as lithium chloride possess relatively high conductivity as they are almost completely dissociated and, hence, may be termed strong electrolytes. Such aqueous electrolytes are conductive at comparatively low temperatures, and are presently used in tantalum electrolytic capacitors. The presence of H2O in such electrolytes limits their use at elevated temperatures due to the high rate of vaporization of Water near and above the boiling point of such electrolytes.

The number of electrolytes which can be employed in tantalum electrolytic capacitors is not unlimited. Many aqueous electrolytes such as solutions of strong bases or salts of strong bases and weak acids are not suitable since during use they Will corrode the anode. In general, it may be stated that prior electrolytes employing nonaqueous solvents are not suited for use With tantalum electrolytic capacitors having Wide range of operating temperatures because of any of several reasons. Frequently, salts are not -suiiiciently soluble in these nonaqueous solvents in order to provide the conductivity required. In many cases these non-aqueous solvents enter into undesired reactions with the solutes employed.

In order to provide tantalum electrolytic capacitors capable of being used over a wide range of temperatures, efforts have been made to develop electrolytes based upon ice eutectic or similar mixtures. In general electrolytes of this type have not proved satisfactory since tantalum electrolytic capacitors in which they are used do not have the desired electrical performance characteristics. At the present time simple aqueous electrolytes are usually employed with tantalum electrolytic capacitors, since best results are achieved With them.

The use of tantalum capacitors is definitely limited by these electrolytes. At the present time common tantalum electrolytic capacitors of the so-called foil type are limited to use at voltages of up to about 150 volts; those of the common sintered, porous pellet type are limited to use at voltages of up to about Volts because of the breakdown or sparking limitations of sulphuric acid electrolytes employed in them. When concentrated aqueous solutions of lithium chloride are employed as electrolytes in tantalum electrolytic capacitors, these units have a limiting voltage rating of approximately 70 volts due to sparking of the electrolyte. The values given here are primarily employed for comparative purposes; those familiar with this field will recognize that these are not exact values.

When an electrolytic capacitor is operated at a high voltage scintillation will take place, destroying part of the oxide film on the anode necessary to the operation of the unit. Within tantalum electrolytic capacitors this damage cannot be repaired as easily as with aluminum, and it results in the leakage current between the electrodes increasing. Even in an undamaged electrolytic capacitor using an aluminum or a tantalum anode some leakage current will flow between the electrodes. Any part of such leakage current flowing through electrolytes of the types previously employed in tantalum electrolytic capacitors will cause such electrolytes to dissociate, forming gas.

The problem of gas formation within any electrolytic capacitor is a serious one. If gas accumulates Within any hermetically sealed capacitor, in time the capacitor will break, allowing the electrolyte to escape and allowing contaminants to enter the unit. The strong electrolytes employed in tantalum electrolytic capacitors will normally cause serious corrosion damage if leakage occurs. Experience has proved that various types of vents designed to permit the escape of gas are not completely satisfactory for use in electrolytic capacitors because such Vents are subject to leakage. At the present time hermetically sealed units are desired for the majority of applications.

A basic object of this invention is to provide new and improved electrolytic capacitors utilizing a specific type of electrolyte as herein defined. A related object of the invention is to teach the composition of a new and improved electrolyte for use in electrolytic capacitors. A further object of this invention is to provide a novel method for manufacturing tantalum capacitors utilizing this electrolyte.

All of these yformal objects of this invention are aimed at certain specific desired results. `One of these objects is to pro-vide tantalum electrolytic capacitors having higher voltage ratings than previously possible. 'Another is to provide tantalum electrolytic capacitors capable of being employed or used over a lgreater range of operating temperatures than previously possible. A further object is to provide an electrolyte which will not form gas Within an electrolytic capacitor by electrolysis of the electrolyte. A still further object is to provide electrolytic capacitors having comparatively low leakage currents.

All of lthese objectives or results achieved with this invent-ion are interrelated, and those skilled in the field to which the entire concept of this invention pertains will realize that luhey may be stated in several different manners. As an aid to understanding the invention, the invention itself can similarly be briefly summarized in several ways. It concerns tantalum electrolytic capacitors which are formed by rst anodizing tantalum anodes, then polarizing these anodes, and then assembling them in electrolytic capacitors using a novel type of electrolyte as herein described. This type of electrolyte contains at least one heterocyclic compound and at least one aliphatic acid or aliphatic acid anhydride. Obviously various equivalent derivatives of such compounds can be employed. As hereinafterv indicated, such electrolytes may either be of a viscous nature or may be liquid.

In order to aid in understanding the invention, there A is shown in the accompanying drawing:

A side elevational view, partially in section, o'f a tantalum electrolytic capacitor of the present invention.

For convenience, the various parts of this drawing are designated by their common names instead of numerals since comparatively few parts are involved in this electrolytic capacitor. Those skilled in the art to which this invention pertains will realize that this invention is not restricted to any particular type of tantalum electrolytic capacitor construction. Thus, the teachings of this specification may be employed with electrolytic capacitors usin-g etched or unetched foil or wire tantalum anodes, or sintered porous tantalum anodes. Various types of known cathodes and sealing means may similarly be employed with the invention.

In producing tantalum electrolytic capacitors having the characteristics indicated above the preparation of 'the anodes employed fis limportant. Initially these `anodes are formed or electrolytically coated with an adherent oxide lm inaccordance with conventional Iprocedures in a weak acidic electrolyte. A taqueous solution of acetic acid may conveniently be used for this purpose or any dilute mineral acid or salts thereof. Other known electrolytes containing ethylene glycol and either mineral or organic acids may also be employed for this purpose; best results have been achieved When these anodes have been found using voltages of from about 100 to about 400 volts.

After an anode has been formed as indicated above so as to have an oxide film, this anode is preferably polarized Vuntil the leakage current through the oxide film becomes constant. Preferably the anode is rinsed prior to polarization in order to avoid carrying over the 'forming electrolyte into the electrolyte employed during this second step in preparing the anode. 'I'his polarization may be carried out in an aqueous mineral acid such as nitric acid, phosphoric acid or the like in concentrations of from about 1% to about 85%. Good results have been obtained with sulphuric acid, especially H2804 solutions. cathode employed during such polarization should, of course, be inert. A potential of at least 75. volts must be employed during such polarization for effective results to be obtained; the voltage employed must also be below the voltage at which scintillation will occur to avoid damaging the oxide iilm.

After these steps an anode for use with this invention is cleaned and dried and then assembled so `as to form a complete electrolytic. capacitor, such as the capacitor shown in the drawing. Here there is shown a tantalurn electrolytic capacitor having a porous tantalum pellet type of anode treated as indicated 4in the preceding located within a silver can serving as a cathode. An inert glass or ceramic washer is fused to both the cathode and to a wire. lead extending from the anode so as to hold the anode within the interior of the cathode and so as to completely seal this capacitor. As electrolyte as herein described is positioned within the cathode. Those skilled in the art to -which fthe invention pertains will realized that the precise method of assembling any electrolytic capacitor of this invention will, of course, vary with the type of anode, containers, etc., employed. It is important however, to assemble the unit utilizing an electrolyte of the type to Which this invention pertains.

Electrolytes `of the type to which this invention pertains are lformed by mixing together at least one aliphatic acid The y or acid anhydride or aryl halide derivative and at least one heterocyclic compound in which the heterocyclic atom is nitrogen so as to form a composition which is liquid or semi-liquid or viscous over the temperature range desired for the electrolytic capacitor being manufactured. For the present day specifications [this temperature range is from -60 c'C. to +150 1C.

It is not necessary that the individual ingredients of such Van electrolyte be liquid or viscous over the entire operating temperature range desired. It is, however, irnportant that the nal composition be liquid or viscous over this range. At the present time it is believed that when the ingredients of Ian electrolyte of this invention are intermixed that a mixture is formed which has properties which .are diiferent trom the properties of the individual ingredients. There is reason to believe that such a mixture may actually consist of certain organic complexes formed between the acidic and the heterocyclic ingredients employed. Satisfactory electrolytes have been created utilizing from about l0 to .about 96 mole percent acidic ingredients and the balance heterocyclic ingredients.

These electrolytes can be formed using liquid aliphati acids such as formic, acetic, proponic, butyric, valerie and heptoic or similar yacids having up to the same number of carbon atoms having branched chains. Also, the alpha substituted halide derivatives of these acids such as alphachloroacetic acid or the anhydrides of any of these acids can be employed. At least one of such acids or anhydrides must be employed 'with the invention; if desired a mixture of them can be utilized.

A large number of different heterocyclic compounds can be employed in the electrolytes of this invention. At least one of such compounds must be employed; if desired a mixture of them can be utilized.A Suitable heterocyclic compounds having one or more six membered rings are pyridine, alpha-, beta, or gamma-picoline, quinoline, isoquinoline lutidines, etc.; some of these compounds are bicyclic, and, hence, include at least one six membered heterocyclic ring. Suitable compounds having tive mem.- bered rings are pyrrole, indole, etc. Compounds such as thiazole containing other elements besides nitrogen and carbon in the ring structure are suitable for use within the invention. Compounds such as imidazole which contain several nitrogen atoms in the ring structure are also suitable provided these nitrogen atoms are not positioned immediately adjacent to one another within the ring structure.

Compounds such as are indicated in the preceding are primarily useful where it is desired to obtain an electrolyte of a liquid character. An important feature of the present invention lies in the discovery that various vinyl derivatives of such compounds, such as various vinyl pyridines, vinyl picolines, vinyl quinolines, pyrroles, indoles, etc, can be utilized to form satisfactory electrolytes of a viscous, semi-solid character. In forming electrolytes having these physical characteristics, the acidic ingredient or ingredients employed are mixed with a vinyl compoundV or a mixture of vinyl compounds and the mixture is polymerized so as to produce the final electrolyte desired. Such polymerization can be conveniently carried out using conventional vinyl polymerization catalysts such as hydrogen peroxide. With polymers such as are created by this procedure, the acidic ingredient or ingredients employed apparently exert a plasticising influence on the cornplete polymer created.

It may be stated that all of these heterocyclic compounds contain either a fiveor six-membered ring in which nitrogen is a heterocyclic nitrogen atom located between two carbon atoms. Obviously the above listing of heterocyclic compounds is not to be taken as being complete. An extremely large number of equivalent compounds may be employed in which inert groups such as methyl, ethyl, propyl, or isopropyl groups are attached to the carbon atoms Within the ring structure groups.

As a specific example of the invention, a porous tantalum anode was formed to 250 volts in a aqueous solution of acetic acid. After being formed it was rinsed and was polarized until a constant leakage current was obtained in a 25% aqueous solution of sulphuric acid at 280 volts. The anode was thencleaned, dried, and then assembled in a complete capacitor utilizing a known type of inert cathode so that the anode and cathode were connected by an electrolyte containing 21.6 mole percent pyridine and`78.4 mole percent glacial acetic acid. This capacitor could be used at voltages above the polarizing voltages and could be operated over a range of temperatures of from about -60 C. to +150 C. In addition, it had extremely low leakage current, and when used over a prolonged period showed no tendency towards gas formation within the electrolyte.

The fact that electrolytes of the present invention do not form gas when used is considered to be extremely important with the instant invention. The precise mechanism of the electrode reactions involved using the electrolytes of this invention is not known. It is considered that current which ows through this electrolyte as leakage current, is probably hole current or electronic current and that the ions which enter into the conduction of the electrolyte merely give up or add electrons at the electrodes. No evidence has been found of reactions producing gaseous products. It is to be emphasized that the above is given by way of theory only and the precise mechanism by which an electrolyte of this invention functions is not presently understood.

Those skilled in the art to which this invention pertains will realize that tantalum electrolytic capacitors formed as herein indicated differ from prior units in their performance characteristics inasmuch as they may be operated at voltage rating substantially above the voltage ratings at which prior tantalum units as indicated in the initial portion of this specification may be operated. Thus, with this invention tantalum electrolytic capacitors having a 300 volt or higher voltage rating may be created. In general it may be stated that any tantalum electrolytic capacitor formed as herein indicated will withstand 25%- 30% higher voltages than the voltages applied to the tantalum anodes during the polarization of these anodes as indicated in this specification. Obviously these percentages are approximate and are used to designate a so-called overload factor which may be considered in connection with the capacitors of this invention.

It is not considered necessary in .this specification to set forth a long list of every possible electrolyte falling Within the scope of this disclosure. Electrolytes containing 1() mole percent acetic acid and 90 mole percent pyridine may be satisfactorily created in the obvious manner. Similarly, electrolytes containing 96 mole percent glacial acetic acid and 4 mole percent pyridine may be created in this manner. Other similar electrolytes using the compounds indicated in the preceding discussion may be made by mixing these compounds Within the ranges of proportions indicated, As an example of this an electrolyte of the present invention was formed using a cornmercial mixture of quinoline and isoquinoline containing about 90% by weight quinoline and about 10% by weight isoquinoline in the amount of by weight and the remainder glacial acetic acid by mixing these two ingredients. A viscous, semi-solid electrolyte of this invention was created by mixing 20 mole percent acetic anhydride with 80 mole percent 4vinyl pyridine and polymerizing this mixture by adding a 35% aqueous solution of hydrogen peroxide in the amount of 10% by weight of the mixture.

A viscous, semi-solid electrolyte of the present invention can, of course, be similarly prepared using proportions of the Various compounds indicated in the preceding with various other types of polymerization catalysts. All the ingredients of such electrolytes are preferably first mixed, and this mixture may then be located against the complete oxide surface of the anode employed so as to ll the space between the anode and the cathode employed. Then, this mixture can be polymerized in situ against a formed anode. In many cases it is considered preferable to employ an acid anhydride in an electrolyte of this invention since the acid anhydride used will react with any uncombined water present so as to produce an anhydrous mixture. Electrolytes as herein described may be employed with various types of electrolytic capacitors besides tantalum electrolytic units.

Because of the nature of this invention, and the fact that it has many aspects, this invention is to be considered as being limited only by the appended claims forming a part of this disclosure.

I claim:

1. A process for manufacturing a tantalum electrolytic capacitor, which comprises: forming an adherent tantalum oxide layer upon a tantalum anode at a voltage of from about l0() to about 400 volts in a weakly acidic electrolyte; polarizing said tantalum anode in an aqueous mineral acid electrolyte at a voltage of from about volts to less than the voltage at which scintillation occurs until a constant leakage current is achieved; cleaning and drying said anode; and assembling said anode in electrolytic capacitor utilizing a cathode and an electrolyte electrically connecting said anode and said cathode.

2. A process for manufacturing a tantalum electrolytic capacitor, which comprises: forming an adherent tantalum oxide layer upon a tantalum anode at a voltage of from about 100 to about 400 volts in a weakly acidic electrolyte; polarizing said tantalum anode in an aqueous mineral acid electrolyte at a voltage of from about 75 volts to less than the voltage at which scintillation occurs until a constant leakage current is achieved; cleaning and drying said anode; and assembling said anode in electrolytic capacitor utilizing a cathode and a liquid electrolyte comprising a mixture of at least one heterocyclic compound containing a nitrogen heterocyclic atom located between two carbon atoms, and a compound selected from a group consisting of aliphatic acids, alpha-halogen substituted aliphatic acids, and anhydrides thereof, said electrolyte electrically connecting said anode and said cathode.

3. A process for manufacturing a tantalum electrolytic capacitor, which comprises: forming an adherent tantalum oxide layer upon a tantalum anode at a voltage of from about 100 to about 400 volts in a weakly acidic electrolyte; polarizing said tantalum anode in an aqueous mineral acid electrolyte at a voltage of from about 75 volts to less than the voltage at which scintillation occurs until a constant leakage current is achieved; cleaning and drying said anode; and assembling said anode in electrolytic capacitor utilizing a cathode and an electrolyte comprising a mixture of from about 10 to about 96 mole percent of at least one acetic compound selected from the group consisting of aliphatic acids, alpha-halogen substituted aliphatic acids, and anhydrides thereof, the remainder of said mixture consisting of at least one heterocyclic compound containing a nitrogen heterocyclic atom located between two carbon atoms, said electrolyte electrically connecting said anode and said cathode.

4. A process for manufacturing a tantalum electrolytic capacitor, which comprises: forming an adherent tantalum oxide layer upon a tantalum anode at a voltage of from about 100 to about 400 volts in a weakly acidic electrolyte; polarizing said tantalum anode in an aqueous mineral acid electrolyte at a voltage of from about 75 volts to less than the voltage at which scintillation occurs until a constant leakage current is achieved; cleaning and drying said anode; and assembling said anode in electrolytic capacitor utilizing a cathode and an electrolyte comprising a mixture of from about 10 to about 96 mole percent glacial acetic acid and from about to about 4 mole percent pyridine, said electrolyte electrically connecting said anode and said cathode.

(References on following page) References'Cited' in the le of this patent UNITED STATES PATENTS Siegmund July 12, 1932r Robinson Nov. 21, 1933 Rhodes Nov. 5, 1935 Lilienfeld Nov. 19, 1935 Owen June 15, 1937 Clark Aug. 10, 1937 8 Brennan Ian'. 11,1938" Robinson et al June 28, 1938 Schimkus Feb. 7, 1939 Georgiev Apr. 18, 1939 Harding July 4, 1939 Clark Aug. 1, 1939 Taylor Feb. 6, 1945 Ruscetta Sept. 4, 1951' Ruscetta et al Mar. 20 1956 

1. A PROCESS FOR MANUFACTURING A TANTALUM ELECTROLYTIC CAPACITOR, WHICH COMPRISES: FORMING AN ADHERENT TANTALUM OXIDE LAYER UPON A TANTALUM ANODE AT A VOLTAGE OF FROM ABOUT 100 TO ABOUT 400 VOLTS IN A WEAKLY ACIDIC ELECTROLYTE; POLARIZING SAID TANTALUM ANODE IN AN AQUEOUS MINERAL ACID ELECTROLYTE AT A VOLTAGE OF FROM ABOUT 75 VOLTS TO LESS THAN THE VOLTAGE AT WHICH SCINTILLATION OCCURS UNTIL A CONSTANT LEAKAGE CURRENT IS ACHIEVED; CLEANING AND DRYING SAID ANODE; AND ASSEMBLING SID ANODE IN ELECTROLYTIC CAPACITOR UTILIZING A CATHODE AND AN ELECTROLYTE ELECTRICALLY CONNECTING SID ANODE AND SAID CATHODE. 